The invention relates to automatic frequency comparison and/or control (AFC) of a phase/frequency of a signal with that of a reference. In a particular embodiment, it is directed to a novel frequency-locked loop architecture which uses gated input and reference signals.
The AFC of the invention will be described in connection with a superregenerative (SRG) radio receiver, though the invention should find other AFC applications. SRG radio receivers are used widely for reception of RF (radio frequency) signals because of its circuit simplicity and exceptionally high signal sensitivity. SRG radio receivers require a stable tuned circuit which is realized by the used of some kind of AFC. It should be noted that frequency and phase can be used interchangeably and the invention can be described by using in either term. It is however decided that xe2x80x9cfrequencyxe2x80x9d is used throughout the present specification.
A block diagram of a simple SRG receiver is depicted in FIG. 1. The input signal VRF is an RF signal at a fixed frequency fRF which is modulated with amplitude-shift keying (ASK); digital data is often transmitted as binary data. VRF is connected to some kind of resonant circuit with natural center frequency fRES. This resonant circuit is connected to some kind of Q control circuit such that overall resonator Q is variable, controlled by a deterministic signal Vq. In FIG. 1, the resonant circuit is an LC tank circuit with the parallel conductance g0 representing losses in the passive components, i.e., finite tank Q, and a transconductor gm configured as a tunable negative resistor, but other kinds of circuit (e.g., a ring oscillator) are possible. The resonator output voltage VRES is connected to an envelope detector followed by a band pass filter (BPF), the combination of which produces a voltage VOUT which contains the demodulated data.
Operation of the SRG receiver just described is as follows. The Q control circuit is driven with a periodic voltage (period Tq, frequency fq=1/Tq) such that for the majority of the period, the resonator is made very slightly unstable. VRES begins to oscillate, building in amplitude with an envelope whose shape is exponential; if left to build for a sufficient period of time, the shape ceases to be exponential because it eventually succumbs to an amplitude limiting mechanism. Usually such a mechanism is inherent to the SRG, for example nonlinearity in the Q control circuit or finite power supply voltage. For the remaining small portion of Tq, the Q control is driven such that the resonator is very stable, to the point that any built-up oscillations rapidly die out and VRES falls to near zero. This is known as xe2x80x9cquenching the oscillationxe2x80x9d. The quench frequency must be at least twice the data rate as the quenching is a form of sampling of the data and the sampling rate is governed by normal Nyquist restrictions.
The method of data detection at VRF works as follows. During the portion of Tq when VRES is permitted to oscillate, the oscillations commence because of wideband thermal noise inherent to any practical electrical system; this creates a nonzero voltage on VRES, and positive feedback inside the resonator passband ensures noise components at frequencies close to fRES are amplified more than those at other frequencies. As a result, the frequency of the built-up oscillation immediately prior to quenching is the same as the natural center frequency of the resonator. If there is coherent energy at the RF input VRF (i.e., a carrier) that falls within the passband of the resonators then the build-up of resonator oscillations is encouraged: the initial voltage at VRES has a larger magnitude than it would if only noise were present. The time constant of the exponential build-up is determined by the setting of Vq alone, and so it is the same whether or not passband energy on VRF is present; the more energy present on VRF, therefore, the greater the final amplitude of the oscillation. Thus, the energy build-up on VRES is proportional to the passband power on VRF and an envelope detector/BPF combination is sufficient to generate a signal proportional to this latter quantity. The envelope detector maintains the subcarrier position of the data relative to the carrier while translating the modulated signal to baseband. Hence, unmodulated signals translate to dc and are rejected by the baseband BPF, while the data pass through the BPF. Of course the BPF must be appropriately matched to the data frequency.
FIG. 2 illustrates typical waveforms and waveform envelopes during the operation of the receiver, all being time coordinated. Vq represent a periodical control signal for quenching. VRES envelope shows exponential build-ups and decays of oscillation amplitude.
Usual embodiments of SRG receivers use a resonator with a fixed center frequency. They require this center frequency to be set fairly precisely because the resonator passband is very narrow during operation (this improves sensitivity to weak RF signals). Component tolerances are often sufficiently poor that fRES can only be set with enough accuracy via mechanical means (for example, trimming the value of a tunable capacitor by hand) prior to first use of the receiver. Thereafter, fRES cannot be changed over the life of the circuit unless it is recalibrated. Manual setting of the resonator frequency is error-prone and expensive; more seriously, fRES might drift after calibration, for example, due to component aging or temperature variations. As noted above, the Vq control is operated such that when the resonator is made to oscillate, the band of frequencies to which the oscillator is sensitive is very narrow; even a slight frequency drift can be enough to mean the transmitted data on VRF is no longer within the resonator passband, which means the SRG receiver is no longer capable of receiving the data.
The present invention addresses these problems by the use of the automatic phase/frequency control in general. Its basic concept is applicable to a variety of areas in which phase/frequency control is needed. The invention, however, is described in detail in connection with the SRG in which the resonator center frequency is made tunable (for example, by replacing the capacitor in an LC-based resonator with a varactor), and the AFC of the invention is provided for adjusting fRES with no external mechanical control. Moreover, the invention provides a means of controlling fRES during receiver operation so that the center frequency is held fixed, even in the face of mechanisms which would otherwise cause the frequency to drift.
Other advantages, objects and features of the present invention will be readily apparent to those skilled in the art from a review of the following detailed description of preferred embodiments in conjunction with the accompanying drawings and claims.
Briefly stated, the invention is directed to a frequency comparison circuit for comparing frequencies of a variable signal and a reference signal. The frequency comparison circuit includes a phase frequency detector for comparing frequencies of the variable signal and the reference signal and for generating an output that depends upon the relationship of the frequencies; and gate modules at the inputs of the phase frequency detector for gating the variable signal and the reference signal so that only a portion of the variable signal and reference signal are repetitively applied to the inputs of the phase frequency detector for processing.
In accordance with another aspect, the invention is directed to a frequency lock loop circuit for maintaining a predetermined frequency relationship between a detected signal and a reference signal. The frequency lock loop circuit includes a variable tuning circuit for adjusting its tuning frequency to generate the detected signal and a frequency comparator for comparing the frequencies of the detected and reference signals and for generating an output responsive to their frequency relationship. The frequency lock loop circuit further includes a gate module for applying repetitively and coincidentally only a predetermined portion of the detected signal and the reference signal to the frequency comparator for generating the output, and a loop circuit connecting the variable tuning circuit and the frequency comparator for applying the output to the variable tuning circuit to adjust its tuning frequency so that the predetermined frequency relationship is maintained between the detected signal and the reference signal.
In accordance with yet a further aspect the invention is directed to an RF receiver of the superregenerative type for detecting an amplitude shift keyed RF signal at a predetermined frequency. The RF receiver includes an RF amplifier for amplifying the RF signal, an RF tuning circuit connected to the RF amplifier and tuned to the RF signal at the predetermined frequency for generating a detected signal, and a quenching circuit attached to the RF tuning circuit for quenching periodically the oscillation of the RF tuning circuit. The RF receiver further includes an envelope detector connected to the RF tuning circuit for producing amplitude shift keyed data carried on the detected signal, a reference signal generator for generating a reference signal, and a loop module connected to the RF tuning circuit and the reference generator for generating a locking output in response to frequency relationship between the reference signal and the detected signal. In the RF receiver, there are provided further a frequency adjusting circuit connected to the RF tuning circuit for adjusting the predetermined frequency to lock it to the reference frequency, and a gate module for sampling the detected signal and the reference signal for a portion thereof to be inputted to the loop module for generating the locking output.